1. Field of the Invention
The present invention relates to circuit protection technology for protecting a circuit to be protected, from surge voltages and the like.
2. Description of the Related Art
Many semiconductor integrated circuits are used in various electronic devices, starting from mobile telephones, PDAs (Personal Digital Assistants), and laptop personal computers, or electrical systems in automobiles. Since usage in all kinds of conditions is envisaged for such semiconductor integrated circuits, high reliability is required. In order to improve reliability, in general, a protection circuit is provided for each bonding pad of an input-output terminal connected to the outside of a circuit.
Among such protection circuits, a voltage clamp circuit may be provided so that the reliability of an internal circuit that is to be protected (referred to as a protected circuit, below), does not deteriorate, even in cases in which a surge voltage or the like is precipitously applied. Circuit protection technology by this kind of voltage clamp circuit is described, for example, in Patent Document 1.
Here, as an example, a protection circuit of a driver circuit 200 of an LED (Light Emitting Diode) shown in FIG. 1 is examined. The LED driver circuit 200 is, for example, a circuit for driving the LED 24 provided as illumination of a meter of an automobile. A battery voltage outputted from a battery 20 is applied via a resistor 22 to an anode of the LED 24. Furthermore, for the cathode of the LED 24, there is a connection to a drive transistor M1 of the LED driver circuit 200. A controller 26 controls gate voltage of the drive transistor M1, and, by regulating current flowing in the LED 24, controls emitted light intensity of the LED 24.
Since voltage outputted from the battery 20 is unstable, for a semiconductor integrated circuit used for this type of application, reliability is required particularly against surge voltages and the like. At the same time, in the LED driver circuit 200 of FIG. 1, breakdown voltage of the drive transistor M1 becomes a problem. Accordingly, a protection circuit 100, which clamps voltage applied to a drain of the drive transistor M1, is arranged in parallel to the drive transistor M1.
Patent Document 1: Japanese Patent Application, Laid Open No. H6-140576
FIG. 2 is a plan view of the LED driver circuit 200 of FIG. 1 seen from above. The LED driver circuit 200 is integrated on a semiconductor substrate 30, and the semiconductor substrate 30 is mounted on a base 32 for a package. A protected circuit 110 integrated on the semiconductor substrate 30 includes a drive transistor M1 of FIG. 1. With regard to the drive transistor M1, which is the protected circuit 110, since it is necessary to inspect breakdown voltage in a state in which the protection circuit 100 is not used, that is, as a single unit, a dedicated bonding pad 34 is provided. The protection circuit 100 also is provided with another bonding pad 36, and the protection circuit 100 and the protected circuit 110 are connected to one another by bonding wires W1 and W2, via a bonding pad 38 arranged on the base 32. The bonding pad 38 is connected to an external electrode of a package, and this external electrode is connected to a cathode of the LED 24 of FIG. 1.
In this way, the protection circuit 100 and the protected circuit 110 are connected via the bonding wires W1 and W2, which are lines that include a significant inductance component.
FIG. 3 is an equivalent circuit diagram of the LED driver circuit 200 of FIG. 1.
The protection circuit 100 is provided with a first transistor Q1, a diode D1, and a resistor R3.
The first transistor Q1 is an NPN bipolar transistor, and is arranged on a path to ground, from the bonding pad 34, which is a connection point of the present protection circuit 100 and the bonding wire W1. The diode D1 is connected between a base and a collector of the first transistor Q1, and the resistor R3 is arranged between the base and an emitter of the first transistor Q1.
In the figure, C1 and C2 represent parasitic capacitances in the LED driver circuit 200; the parasitic capacitance C1 is mainly collector-emitter capacitance of the first transistor Q1 of the protection circuit 100, and the parasitic capacitance C2 is drain-source capacitance of the drive transistor M1 inside the protected circuit 110. Furthermore, the bonding wires W1 and W2 respectively include resistance components R1 and R2, and inductance components L1 and L2. Included in the resistance components R1 and R2 are not only the bonding wires W1 and W2, but also IC chip internal wiring resistance.
When voltage of the bonding pad 38 rises due to occurrence of a surge voltage, voltage Va of the bonding pad 34 rises therewith. When the voltage Va of the bonding pad 34 exceeds a Zener voltage Vz of the diode D1, a reverse current directed from cathode to anode flows, the first transistor Q1 is ON, and a current is extracted from the bonding pad 34. As a result, voltages Va and Vb of the bonding pad 34 and the bonding pad 36 are clamped, and it is possible to prevent application of a high voltage to the protected circuit 110.
With the protection circuit 100 configured in this way, problems described below arise. FIG. 4 is a voltage waveform diagram of the LED driver circuit 200 of FIG. 3, and shows a time waveform of the voltage Va of the bonding pad 34 and the voltage Vb of the bonding pad 36 of FIG. 3.
At time T0, when a surge voltage is inputted from the bonding pad 38, both of the voltages Va and Vb of the bonding pad 34 and 36 rise. When the voltage Va of the bonding pad 34 rises and exceeds the Zener voltage Vz of the diode D1, a reverse current directed from cathode to anode of the diode D1 flows, and the first transistor Q1 is ON.
If base-emitter voltage of the first transistor Q1 is taken as Vbe, the voltage Va of the bonding pad 34 is clamped close to Vmax=Vz+Vbe, as shown in FIG. 4.
However, as shown in FIG. 3, parasitic capacitances of different capacitance values exist for each of the bonding pad 34 and the bonding pad 36. If C1>C2, charge stored in the parasitic capacitance C2 of the protected circuit 110 is extracted by the protection circuit 100. At this time, the charge stored in the parasitic capacitance C2 is discharged by the protection circuit 100 via the bonding wires W1 and W2, which include the inductance components L1 and L2.
By a current flowing in the inductance components L1 and L2, LCR resonance is generated by the parasitic capacitances C1 and C2, the resistors R1 and R2, and the inductance components L1 and L2 included in the bonding wires W1 and W2, and a reverse voltage is generated with respect to the inductance components L1 and L2. As a result, the voltage Vb of the bonding pad 36 rises while oscillating, and the oscillation continues even after time T1 at which the voltage Va of the bonding pad 34 is clamped at the voltage Vmax. As a result, for the protected circuit 110, there have been cases in which voltage exceeding the voltage Vmax is applied, and there has been room for improvement in functionality of the protection circuit 100.